Uses and environmental effects

Solvents that are toxic on exposure. Dichloromethane ($CH_2Cl_2$) is a volatile paint-remover and metal-degreasing solvent, and also a process solvent in drug manufacture and decaffeination; because it evaporates readily, it can accumulate in poorly ventilated workspaces and cause CNS depression (dizziness, headache) with prolonged exposure, and is a suspected carcinogen at heavy repeated doses. Trichloromethane, or chloroform ($CHCl_3$), was once the standard surgical anaesthetic but is no longer used that way because of liver and heart toxicity, and because it slowly oxidises in air and light into poisonous phosgene ($COCl_2$); today it survives mainly as a solvent and as an intermediate in freon manufacture. Tetrachloromethane, or carbon tetrachloride ($CCl_4$), was once a leading industrial solvent, fire-extinguisher fluid, and dry-cleaning agent, but prolonged exposure is highly toxic to liver and kidneys, and — because it is stable enough to survive into the stratosphere — it also contributes to ozone depletion; it has been largely phased out on both counts.

A fading antiseptic. Iodoform ($CHI_3$) was historically dabbed onto wounds as an antiseptic dressing because it slowly liberates iodine on contact with tissue. It has been superseded by modern antiseptics, and its main drawback was never acute toxicity but its strong, persistent smell.

Refrigerants that are safe to touch but dangerous to the sky. Freons — chlorofluorocarbons (CFCs) such as Freon-12, $CCl_2F_2$ — were chosen as refrigerants and aerosol propellants precisely because they are chemically inert, non-flammable, and of low direct toxicity: safe to handle, safe to breathe near. That very inertness is the problem. Freons do not break down near the ground, so they survive long enough to drift upward into the stratosphere, where intense ultraviolet radiation (filtered out lower down) breaks the carbon–chlorine bond and releases reactive chlorine species that catalytically destroy ozone ($O_3$) molecules. The full radical-chain mechanism is beyond this syllabus — only the qualitative freon-to-ozone-depletion link is required.

An insecticide that concentrates as it climbs. DDT (dichlorodiphenyltrichloroethane) is an agricultural and household insecticide, historically important for controlling insect-borne diseases such as malaria. It is chemically stable and fat-soluble, so an organism that absorbs it stores it in fatty tissue rather than excreting it — this build-up within one organism over time is bioaccumulation. Because a predator eats many prey and each carries its own stored DDT, the concentration rises at every step up the food chain — this is biomagnification. The combined effect has caused documented ecological harm, most famously eggshell thinning in birds of prey, and DDT's agricultural use is now banned or restricted in many countries.

There is no formula here — the syllabus asks for a qualitative link between property and environmental effect, not a mechanism. One reasoning pattern explains all six compounds at once, and it is what "explain why" questions test: the property that makes a haloalkane commercially useful is almost always the same property responsible for its environmental or health cost. Dichloromethane's volatility makes it a fast solvent and also lets it accumulate as vapour indoors. Chloroform's usefulness as a solvent and freon precursor coexists with its tendency to oxidise into toxic phosgene. Carbon tetrachloride's solvent power and its stability are the same stability that lets it survive to the stratosphere. Freons are chosen for their inertness — the same inertness lets them survive the journey to the ozone layer intact. DDT's fat solubility and stability make it a long-lasting insecticide, and precisely what let it accumulate in fat and magnify up food chains. Iodoform is a partial exception: its drawback (smell) is a nuisance, not a hazard, so it is treated separately. Reading each compound this way — property gives the use, the same property gives the cost — turns six facts to memorise into one pattern to apply.

A student claims "all six chlorinated compounds in this unit are equally dangerous to the environment, so they should all have been phased out at the same time." Evaluate this claim by comparing dichloromethane, freons, and DDT.

Step 1. Compare persistence and mechanism. Dichloromethane is volatile and comparatively reactive; it does not persist for long in the environment or in living tissue, and its hazard is direct toxicity/CNS depression to people exposed to high vapour concentrations in poorly ventilated settings — a local, short-timescale hazard.

Step 2. Freons are chosen for refrigeration because they are extremely inert at ground level, which is what makes them safe to handle directly, but that inertness lets them survive for years and drift into the stratosphere, where ultraviolet light breaks the C–Cl bond and releases chlorine species that destroy ozone — a global, delayed-onset hazard that has nothing to do with direct toxicity.

Step 3. DDT is chemically stable and fat-soluble, so it persists for years in soil and living tissue, accumulating in fat (bioaccumulation) and becoming more concentrated at each step up the food chain (biomagnification), causing ecological harm well beyond the site and time of application — a persistent, food-chain-wide hazard.

Step 4. Conclusion. The three compounds pose three genuinely different kinds of hazard — acute local toxicity, global atmospheric damage, and persistent ecological accumulation — so the claim that they are "equally dangerous" is an oversimplification. Each compound's environmental effect depends on its own volatility, stability, and fat solubility, not merely on the fact that it is chlorinated.

Competitive stretch

Explain, in outline, why freons threaten the ozone layer even though they are non-toxic and unreactive at ground level, and why this makes them harder to regulate by simple toxicity testing. Step 1. Ground-level toxicity tests would pass freons easily, because their inertness makes them safe to breathe and handle directly. Step 2. That same inertness is what lets them survive years-long transport up into the stratosphere without decomposing near the surface. Step 3. Only in the stratosphere, under intense UV, does the C–Cl bond break and release ozone-destroying chlorine — a delayed, location-shifted effect invisible to any standard ground-level toxicity screen. This is why freons were regulated only after atmospheric monitoring revealed ozone depletion, not because any toxicity test flagged them.

• Assuming "non-toxic to humans" means "environmentally safe" — freons are the textbook counter-example: harmless on direct contact, environmentally damaging via ozone depletion.
• Describing the ozone-depletion mechanism in full photochemical/radical-chain detail — the syllabus needs only the qualitative freon-to-ozone link, not the reaction cycle.
• Confusing bioaccumulation (build-up of a substance within one organism's tissue over its lifetime) with biomagnification (rising concentration at each successive trophic level up a food chain) — DDT shows both, and the two words are not interchangeable.
• Treating all six compounds as having identical uses or identical hazards — each has one specific major use (solvent, anaesthetic-turned-solvent, dry-cleaning agent, antiseptic, refrigerant, insecticide) and one specific type of concern that should not be blurred together in an answer.

Ask the tutor: want a quick compare-and-contrast table of all six compounds' uses versus hazards, or a worked walkthrough of a bioaccumulation-versus-biomagnification food-chain example?